Brownies:

1 cup unsalted butter (plus more for preparing pan)
3 ounces semisweet chocolate, finely chopped
1 cup granulated sugar
1/2 cup cocoa powder
1/2 cup mascarpone cheese, softened
3 large eggs, at room temperature
2 teaspoons vanilla extract
1/2 cup all-purpose flour
1/4 teaspoon salt

Ganache:
6 ounces semisweet chocolate, finely chopped
6 tablespoons heavy cream
3 Tablespoons unsalted butter